Delta's future: rising waters, sinking levees

It is well known that some bowl-like Delta islands have been slowly sinking for decades, but a recent study shows the levees protecting those islands are also subsiding - albeit, at a slower rate.

Alex Breitler

It is well known that some bowl-like Delta islands have been slowly sinking for decades, but a recent study shows the levees protecting those islands are also subsiding - albeit, at a slower rate.

Sinking levees combined with rising sea levels means that by late this century, most Delta levees may no longer meet a flood-protection standard, according to the study by the University of Hawaii. The work is part of a larger package of climate change studies funded by the California Energy Commission and released last week.

The interior of Delta islands has been sinking because of human practices. Farming compacts the organic peat soil, causing a slow drop in land elevation over time.

The rate of sinking has slowed down as farming techniques have improved. However, the levees themselves have also been sinking by a rate of about of 1 to 2 millimeters per year - a fraction of an inch.

And that's at least partly related to a natural phenomenon: the compaction of vast amounts of sediment swept into the Delta over a period of hundreds of thousands of years, said the study's lead author, Benjamin Brooks, a researcher with the university's Institute of Geophysics and Planetology.

"It's just the natural process of a river," Brooks said last week.

The study, using tens of thousands of data points acquired with satellite technology, gives officials a better understanding of regional subsidence in the Delta, Brooks said.

The years 2050 to 2075 will be a "critical time" for Delta levees as the sea level creeps up and the levees continue to sink, the study says.

The stability of the levees is a sensitive issue. State officials and Southern California water stakeholders often use the levees as justification to build large tunnels to divert water away from the estuary, a plan recently championed by Gov. Jerry Brown.

Tunnel opponents say the levees can be strengthened for far cheaper than building tunnels. The new study does not address that levees, sinking though they are, can be raised over time to offset sea level rise, said Robert Pyke, a consulting engineer who supports the levee solution.

"To assume they would not be improved in the face of that rapid sea level rise is ridiculous," Pyke said. "I don't think we're going to see that degree of sea level rise, but if we did people would be scurrying around like crazy" raising the levees.

The "fat levees" proposed by tunnel opponents allow plenty of horizontal room on the crown of the levee to expand upward, Pyke said.

Strangely, the study found that one area of the Delta - Roberts Island, near Stockton - has been rising, not sinking. It's not entirely clear why, but Brooks said it may be related to short-term shifts in elevation caused by seasonal runoff into the Delta.